Signaling system



May 27, 1941.

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29 INVENTOR.

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May 27, 1941.

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Patented May 27, 1941 SIGNALING .SYS TEM Harold 0. Peterson, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application September 13, 1938, Serial No. 229,644

2 Claims.

This invention relates to radio transmitters and, more particularly, to remotely controlled point-to-point radio transmitters.

In many types of communication the transmitter is idle a relatively large percentage of the time. For instance, the average point-to-point radio telephone transmitter is active only during periods when a call is actually being put through. For purposes of maintaining contact it is, however, generally desirable to keep these transmitters operating during the idle periods. This practice also enables the circuit to become operative almost instantly without the practical delays entailed by the necessity for starting up a transmitter from the cold condition. With certain types of receivers, particularly those having automatic frequency control, it is also desirable to maintain a carrier output at the transmitters during idle periods so that the receiver continues to hold itself in tune.

Now if full carrier is radiated all the time during idle periods, the power used and the tube depreciation amounts to a considerable expenditure. It is, however, not necessary to keep the carrier at full amplitude during idle periods in order to maintain automatic frequency control at the receiver, or for that matter to maintain contact over the radio circuit. Whereas the previous practice has been to shut down the transmitter during idle periods, it is an object of my invention to provide means whereby the transmitter carrier is continued on the circuit but at reduced amplitude. Thus, if the transmitter output is reduced on the order of to 20 db. during idle periods, a considerable saving in power may be realized, still preserving the advantages of antomatic frequency control and continuous circuit contact. This saving in power and tube life can be realized in any type of transmitter as, for instance, amplitude or phase modulated transmitters. It is especially useful in the case of frequency or phase modulation since in that type of modulation the carrier output is normally at its maximum value during periods of no modulation.

A further object of my invention is to control the amplitude of a carrier emitted by a radio transmitter from a remote control point.

Still a further object of my invention is to provide means automatically actuated by a signal arriving at a radio transmitter for increasing the power of said transmitter.

Briefly, my invention comprises a variable conductance transducer in the circuit between the oscillator and power amplifier of a radio transmitter. The conductance of said transducer is normally low but in response to a control current, or in response to the arrival of signal currents, the conductance is increased whereby the transmitter is caused to radiate at full power.

A more complete understanding of my invention may be had by reference to the following detailed description which is accompanied by drawings in which Figure 1 illustrates an embodiment of my invention. In this embodiment a remote control switch 2 at the control point operates a relay I3 which controls the transmit ter carrier output level, selecting either full carrier or reduced carrier output. The modification shown in Figure 2 incorporates an automatic device, which may be situated at the transmitting station to cause the transmitter to radiate at reduced carrier amplitude during idle periods. Figure 3 shows another modification of my inven tion for causing the transmitter to automatically radiate at reduced power during idle periods.

Referring to Figure 1, the modulation from the source of signals l at a point remote from the transmitter is brought to the transmitter over a transmission line 4. The balanced transmission line is coupled to source I through a center tapped secondary transformer 5 and to modulation amplifier 6 through a transformer 5' having a center tapped primary. The amplifier transfers the modulation into modulator device I which may be any of the well known types of modulator as for instance amplitude modulator, frequency modulator, or phase modulator. The carrier frequency control device or generator l2 feeds control frequency through a tube H and an amplifier l0 into the modulator 1. The amplifier [0 might also be a frequency changing device such as .a multiplier for instance. It is, however, desirable that the carrier amplitude radiated by the transmitter varies with change of transconductance of vacuum tube l. The transconductance of vacuum tube H is controlled by change of its grid voltage when relay I3 causes resistance I6 to be cut in andout of circuit. Relay I3 is connected to the transmission line 4 through the center tap of transformer 5'. At the remote control point switch 2 and battery 3 are connected to the center tap of transformer 5 so that the carrier output from the transmitter may effectively be controlled by the operation at the control point. The return for the control circuit may be through ground. A separate control circuit may be used but the circuit shown is preferable because of line economy.

In Figure 2 the incoming modulation over line 4 is caused to operate a device which changes the level of the radiated carrier automatically. The circuit is such that the first few milliseconds of the incoming modulation will change the condition of the transmitter to give full carrier output and full carrier output is maintained continuously until after the modulation has ceased for a pre-determined period of time determined by the time constant circuit 25, 26. Reference numeral 20 represents a delay network which may be inserted in series with the modulation going to the transmitter through amplifier 6 so that modulation does not reach the modulator I until after the control circuit has had time to adjust the transmitter power to full output. The output of modulator I, as in the previous example, passes through power amplifier stages 8 to antenna 9. The carrier level from frequency control I2 is regulated by vacuum tube II feeding into the modulator I through transformer 2|. Reference numeral 22 represents an amplifier which may also contain frequency changing means such as for instance, multipliers. In operation, the incoming modulation is passed through amplifier 23 into rectifier 24 which is arranged so that when the incoming modulation exceeds a pre-determined level, current flows through rectifier 24 and resistance 25, effectively reducing the plate current in vacuum tube 21 substantially to zero, thereby reducing the voltage drop in resistance 28 to zero, which condition is favorable to maximum transconductance through vacuum tube I I. This in turn results in radiation of carrier at full amplitude. This condition of carrier at full amplitude is continued for a pre-deterrnined time even in the absence of modulation by the resistance-capacity, time constant circuit 25, 26. Appropriate biases and plate voltages are supplied to tube 21 from direct current source 29 through a voltage divider consisting of resistances 30 and 3i. Resistance 32 supplies fixed bias for vacuum tube I I. Condenser 33 functions as a by-pass condenser.

In Figure 3 a difierent automatic control circuit is shown from that in Figure 2. The modulation arrives at the transmitting station over transmission line 4 where it is divided between two branch circuits one of which feeds through a delay device 20, and amplifier 6 into modulator I. The other branch operates the "automatic carrier level control device. This device consists of an amplifier 23 impressing voltage upon the control electrode of a gaseous discharge tube 34. The nature of this discharge device is such that when voltage from transformer 35 impressed on control electrode 36 exceeds a certain value the gas within is ionized resulting in the How of current between electrode 39 and electrode 38. This flow within the discharge tube places a negative bias upon the grid of vacuum tube 21 which reduces the voltage drop in resistance 28 to substantialy zero, thus, as in Figure 2, increasing the transconductance of vacuum tube I I and allowing the passage of full carrier output. In the absence of modulation, the time constant circuit 25, 26 serves to maintain the carrier radiated at full value for a pre-determined length of time.

Modulator I may be any of the well known types of modulating devices such as for instance, amplitude, frequency or phase modulators.

My invention may also be applied in other ways to the control of carrier output. For instance, in some transmitters it may be found more desirable to control the radiated carrier by changing the transconductance of some of the power stages following the modulator. For example, the transconductance of the tubes in power amplifier 8 may be controlled in the same way as shown for tube II.

The type of modulation may be any of the types commonly used for communications purposes as for instance, telephony, facsimile, or telegraph modulation.

While I have shown and particularly described several embodiments of my invention, it is to be distinctly understood that my invention is not limited thereto but that modifications within the scope of my invention may be made.

I claim:

1. In a radio transmitter a source of carrier energy, a source of signal energy, means for modulating said carrier by said signal, a delay network having its input connected to said source of signal energy and its output to said modulator, and means interposed between said source of carrier energy and said modulator for controlling the amplitude of said carrier, including a thermionic discharge tube having at least a plate and a grid, said grid being connected to said carrier source and said plate coupled to said modulator, means for normally applying a bias to said grid of such value that only a portion of said carrier energy is applied to said modulator, a thermionic discharge tube having a plate circuit connected across a portion of said biasing means and a grid circuit, a gas discharge tube having an input circuit and an output circuit connected thereto, said input circuit being connected to the input circuit of said delay network andv said output circuit being so connected to the grid circuit of said second discharge tube that when said input circuit is energized the bias on the grid ofv said first named discharge tube is decreased whereby carrier energy of increased amplitude is applied to said modulator.

2. In a radio transmitter a source of carrier energy, a source of signal energy, means. for modulating said carrier by said signal, a delay network having its input connected to said source of signal energy and its output to said modulator, and means interposed between said source of carrier energy and said modulator for controlling the amplitude of said carrier, including a thermionic discharge tube having at least a plate and a grid, said grid being connected to said carrier source and said plate coupled to said modulator, a plurality of serially connected resistors, a source of potential across said resistors, said grid being so connected to said resistors that said tube is normally biased substantially to cut-off whereby only a portion of said carrier energy is applied to said modulator, a thermionic discharge tube having a plate circuit connected across at least one of said resistors and a grid circuit, means for applying a direct potential to said grid circuit in response to signals in the input circuit of said delay network whereby the biassing on said first tube is decreased and carrier energy of increased amplitude is applied to said modulator.

HAROLD O. PEIIERSO-N. 

